ISDN is a general purpose digital network capable of supporting access to a wide range of interconnected services, such as voice, data, facsimile and video. ISDN achieves the support of a large variety of services by providing a standard digital network-user interface. A standardized interface, including different configurations, was disclosed in the I.400 series of recommendations by the CCITT in 1984.
Currently, there are two recommended standard ISDN interfaces for user access. They include a basic rate interface and a primary rate interface. A basic rate S/T interface supports transmission at a rate of 192 kbps. As recommended by the CCITT, a single ISDN basic rate S/T interface can provide a variety of voice and data services and accommodate up to eight terminal equipments.
As illustrated in FIG. 1, a standard user-network interface comprises a public network circuit 10 (e.g., a central office switch or a PABX) and a network termination unit (NT) 11 which terminates a trunk line (digital subscriber bus) 13 from the public network circuit 10. A plurality of terminal equipments (TEs) 21, 22, . . . , 28 are connected through a passive S interface bus (or S-bus) 12 in a point to multipoint configuration which supports two-way communication between the NT 11 and the individual TEs 21, . . . , 28. The signal transmission direction from the NT 11 towards the TEs 21, 22, . . . , 28 is denoted by 12a and the signal transmission direction from the TEs 21, 22, . . . , 28 towards the NT 11 is denoted by 12b.
An ISDN, basic rate interface provides two full-duplex 64 kbps channels for communication, designated B1 and B2, and one full-duplex 16 kbps channel for signalling, designated D. The B-channels are the fundamental user access channels and are used to transmit circuit-switched type communication information data, digital, voice, or a mix thereof. The D-channel carries signalling information that controls circuit-switched calls on the associated B-channels. Such circuit-switched signalling is described in William Stallings, ISDN An Introduction ch. 8-5 (1989). In addition, the D-channel may be used for packet-switching or low-speed data transmission when the channel is not used for signalling.
The aforementioned channels and other signals transmitted in each direction 12a, 12b are combined into digital bit stream signals according to the standard ISDN S/T interface frame formats defined in the CCITT recommendations. FIG. 2 illustrate the standard frame formats for signal transmission on the S/T interface bus 12a and 12b respectively. The frames for the two directions both last 250 .mu.s and are transmitted at a frame rate of 4kHz. FIG. 2 depicts the frame format for frames which are illustratively sequentially broadcast from the NT 11 to the TE's 21, . . . , 28 in the direction 12a. Likewise, FIG. 2 also depicts frames broadcast from TE's 21, . . . , 28 to the NT 11 in the direction 12b . Illustratively, frames transmitted by the TEs 21, . . ., 28 in the direction 12b lag the frames transmitted by the NT 11 in the direction 12a by two bits.
AMI (alternative mark inversion) codes are used to transmit binary information on the S/T interface bus wherein a binary one is represented by no pulse and a binary zero is represented by a positive or negative pulse. Possible electric levels of each bit in the frames are shown in FIG. 2. In order to maintain a DC balancing scheme on the S/T interface, the number of negative pulses of each frame must equal the number of positive pulses.
Although the two frame formats transmitted in the two opposite directions 12a and 12b on the S/T interface are not symmetrical, both contain forty-eight bits including four D-channel bits (D) and four alternating octets of B1 and B2-channel bits, denoted as B1 and B2 respectively. Other control bits are included for framing (F), DC balancing (L), etc.
In order to maintain the DC balancing scheme according to the CCITT standard frame format, TE's may not share an octet of B-channel bits. When one TE occupies a whole octet of B-channel bits during communication, the other TEs connected with the same S/T interface bus must keep this B-channel idle, i.e., the other TEs transmit an octet of bits with the binary logic value 1, each bit having a zero amplitude, on this B-channel. As a result, a TE must occupy at least one entire B-channel, having a bandwidth of 64kHz, for voice or circuit-switched type communication whether or not the entire bandwidth of the B-channel is needed.
Since there are only two circuit-switched communication channels (B1 and B2) available on the basic access S/T interface, a single ISDN S/T interface permits, at most, only two TEs to receive or transmit information simultaneously. A problem arises in such a communication system having up to eight TEs connected to a single basic rate S/T interface, when two TEs are operated simultaneously and a third TE desires to commence immediate operation or is being called. There are no vacant B-channels available for the third TE. Further, each TE is allocated channels in 64 kHz bandwidth increments.
U.S. Pat. No. 4,935,923 (Shimizu), issued on Jun. 19, 1990 discloses an adapter for connecting a plurality of terminal equipments to a network termination unit in an ISDN system. The Shimizu adapter, however, does not allow more than two terminal equipments to simultaneously transmit on the S interface bus. U.S. Pat. No. 4,970,723 (Lin), issued on Nov. 13, 1990 discloses an ISDN basic rate interface system for expanding the number of voice terminals which may be simultaneously operated through the interface to four. The Lin system increases the number of terminals by converting eight-bit voice data to four-bit in order to permit two TEs to share each B-channel.
Specifically, in each subframe in the Lin reference, the B1 channel octet is divided into two four-bit nibbles to form a B11 channel and a B12 channel. The B2 channel octet is divided into two four-bit nibbles to form a B21 channel and a B22 channel. Thus, instead of two B channels each with a slot size of eight bits in a subframe, there are four channels each with a slot size of four bits. In this manner, a total of up to four TE's share the two B channels.
It is therefore an object of the present invention to provide a single ISDN basic rate S/T interface supporting more than two circuit-switched communication channels on a single S interface bus.
It is another object of this present invention to permit simultaneous multiple communications among a plurality of TEs and a public network circuit using a single basic rate S/T interface without modifying basic standard frame structures on the S interface bus.
It is also an object of this present invention to provide an ISDN, basic access user-network interface which supports multiple circuit-switched type communication channels with a narrower bandwidth unit for the TEs to utilize.